The NY Times had an article yesterday about how buying a hybrid may have zero effect on how much oil the US saves.
This is because of the CAFE (corporate average fuel economy) rule, which sets the minimum average gas mileage a manufacturer's product can have. Right now all of a maker's automobiles must average 27.5 miles per gallon, with its light trucks averaging 21.5 mpg. If a manufacturer has a car that averages 26.5 mpg, it must have another car that averages 28.5 mpg to make up for it.
So here's where the hybrids come in: if a hybrid truck averages 26.5 mpg, that allows a manufacturer to have another truck that averages 16.5 mpg. If they can make a hybrid truck that averages 31.5 mpg, they can have another truck that averages 11.5 mpg. So everyone that buys a hybrid with better mileage is essentially allowing someone else to buy a worse gas guzzler. As far as the country is concerned its a zero-sum game because every gallon of fuel you save is consumed by someone else.
This does not appear to be the case with Toyota's Prius (and, I imagine, Honda's hybrids as well). Toyota's cars are above the 27.5 average so buying a Prius does not mean Toyota is going to push the envelope in manufacturing gas-guzzling cars. But not so with Toyota's light trucks & SUVs. They are very close to the 21.5 mpg minimum, so your buying a Highlander hybrid means someone else can buy the Lexus SUV. Ditto for Ford's Escape hybrid; most likely a Lincoln Navigator owner will thank you.
The article offers two ways to really decrease the country's consumption of oil: financial incentives or financial disincentives (i.e., gas tax). I'd offer a third: increase the CAFE minimums.